An improved method of angle measurement is proposed based on a parallel plate interferometer. A position detection system is incorporated into a parallel plate interferometer in order to realize large deflection angle measurement. A reflecting mirror is introduced for increasing the measurement resolution. In experiments, a deflection angle of a measured target was measured within about 3 deg. with high accuracy. And as a phase modulating interferometer, it was used to measure a small angular displacement with a repeatability of 5.5*10^(-8) rad.